System and method for monitoring ablation size

ABSTRACT

A system for monitoring ablation size is provided and includes a power source including a microprocessor for executing at least one control algorithm. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including data pertaining to a control curve varying over time and being representative of one or more electrical parameters associated with the microwave antenna. Points along the control curve correspond to a value of the electrical parameters and the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/750,790, filed on Jun. 25, 2015, which is acontinuation application of U.S. patent application Ser. No. 13/764,386filed on Feb. 11, 2013, now U.S. Pat. No. 9,271,791, which is adivisional application of U.S. patent application Ser. No. 12/607,268,filed on Oct. 28, 2009, now U.S. Pat. No. 8,382,750, the entire contentsof all of which are incorporated by reference herein.

BACKGROUND Technical Field

The present disclosure relates to systems and methods that may be usedin tissue ablation procedures. More particularly, the present disclosurerelates to systems and methods for monitoring ablation size duringtissue ablation procedures in real-time.

Background of Related Art

In the treatment of diseases such as cancer, certain types of cancercells have been found to denature at elevated temperatures (which areslightly lower than temperatures normally injurious to healthy cells).These types of treatments, known generally as hyperthermia therapy,typically utilize electromagnetic radiation to heat diseased cells totemperatures above 41° C. while maintaining adjacent healthy cells atlower temperatures where irreversible cell destruction will not occur.Procedures utilizing electromagnetic radiation to heat tissue mayinclude ablation of the tissue.

Microwave ablation procedures, e.g., such as those performed formenorrhagia, are typically done to ablate the targeted tissue todenature or kill the tissue. Many procedures and types of devicesutilizing electromagnetic radiation therapy are known in the art. Suchmicrowave therapy is typically used in the treatment of tissue andorgans such as the prostate, heart, and liver.

One non-invasive procedure generally involves the treatment of tissue(e.g., a tumor) underlying the skin via the use of microwave energy. Themicrowave energy is able to non-invasively penetrate the skin to reachthe underlying tissue. However, this non-invasive procedure may resultin the unwanted heating of healthy tissue. Thus, the non-invasive use ofmicrowave energy requires a great deal of control.

Currently, there are several types of systems and methods for monitoringablation zone size. In certain instances, one or more types of sensors(or other suitable devices) are operably associated with the microwaveablation device. For example, in a microwave ablation device thatincludes a monopole antenna configuration, an elongated microwaveconductor may be in operative communication with a sensor exposed at anend of the microwave conductor. This type of sensor is sometimessurrounded by a dielectric sleeve.

Typically, the foregoing types of sensor(s) are configured to function(e.g., provide feedback to a controller for controlling the power outputof a power source) when the microwave ablation device is inactive, i.e.,not radiating. That is, the foregoing sensors do not function inreal-time. Typically, the power source is powered off (or pulsed off)when the sensors are providing feedback (e.g., tissue temperature) tothe controller and/or other device(s) configured to control the powersource.

SUMMARY

The present disclosure provides a system for monitoring ablation size inreal-time. The system includes a power source including a microprocessorfor executing one or more control algorithms. A microwave antenna isconfigured to deliver microwave energy from the power source to tissueto form an ablation zone. An ablation zone control module is inoperative communication with a memory associated with the power source.The memory includes one or more data look-up tables including datapertaining to a control curve varying over time and being representativeof one or more electrical parameters associated with the microwaveantenna. Points along the control curve correspond to a value of theelectrical parameters and the ablation zone control module triggers asignal when a predetermined threshold value of the electricalparameter(s) is measured corresponding to the radius of the ablationzone.

The present disclosure also provides a microwave antenna adapted toconnect to a power source configured for performing an ablationprocedure. The microwave antenna includes a radiating section configuredto deliver microwave energy from a power source to tissue to form anablation zone. An ablation zone control module in operativecommunication with a memory associated with the power source. The memoryincludes one or more data look-up tables including data pertaining to acontrol curve varying over time and being representative of one or moreelectrical parameter(s) associated with the microwave antenna. Pointsalong the control curve correspond to a value of the electricalparameter(s) and the ablation zone control module triggers a signal whena predetermined threshold value of the at least one electrical parameteris measured corresponding to the radius of the ablation zone.

The present disclosure also provides a method for monitoring temperatureof tissue undergoing ablation. The method includes an initial step oftransmitting microwave energy from a power source to a microwave antennato form a tissue ablation zone. A step of the method includes monitoringreflected power associated with the microwave antenna as the tissueablation zone forms. A step of the method includes communicating acontrol signal to the power source when a predetermined reflected poweris reached at the microwave antenna. Adjusting the amount of microwaveenergy from the power source to the microwave antenna is another step ofthe method.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a perspective view of a system for monitoring ablation sizeaccording to an embodiment of the present disclosure;

FIG. 2 is a functional block diagram of a power source for use with thesystem depicted in FIG. 1;

FIG. 3A is a schematic, plan view of the tip of a microwave antennadepicted in FIG. 1 illustrating radial ablation zones having a sphericalconfiguration;

FIG. 3B is a schematic, plan view of the tip of a microwave antennadepicted in FIG. 1 illustrating radial ablation zones having anellipsoidal configuration;

FIG. 4A is a graphical representation of a reflected power (P_(r))versus time (t) curve;

FIG. 4B a graphical representation of a corresponding reflected power(P_(r)) versus ablation radii (Ar) curve;

FIG. 4C is a graphical representation of the derivative (dP_(r)/dt) ofthe reflected power (Pr) versus time (t) curve; and

FIG. 5 is a flow chart illustrating a method for monitoring temperatureof tissue undergoing ablation in accordance with the present disclosure.

DETAILED DESCRIPTION

Embodiments of the presently disclosed system and method are describedin detail with reference to the drawing figures wherein like referencenumerals identify similar or identical elements. As used herein and asis traditional, the term “distal” refers to the portion which isfurthest from the user and the term “proximal” refers to the portionthat is closest to the user. In addition, terms such as “above”,“below”, “forward”, “rearward”, etc. refer to the orientation of thefigures or the direction of components and are simply used forconvenience of description.

Referring now to FIG. 1, a system for monitoring ablation size isdesignated 10. The system 10 includes a microwave antenna 100 that isadapted to connect to an electrosurgical power source, e.g., an RFand/or microwave (MW) generator 200 that includes or is in operativecommunication with one or more controllers 300 and, in some instances, afluid supply pump 40. Briefly, microwave antenna 100 includes anintroducer 116 having an elongated shaft 112 and a radiating orconductive section or tip 114 operably disposed within elongated shaft112, a cooling assembly 120 having a cooling sheath 121, a handle 118, acooling fluid supply 122 and a cooling fluid return 124, and anelectrosurgical energy connector 126. Connector 126 is configured toconnect the microwave antenna 100 to the electrosurgical power source200, e.g., a generator or source of radio frequency energy and/ormicrowave energy, and supplies electrosurgical energy to the distalportion of the microwave antenna 100. Conductive tip 114 and elongatedshaft 112 are in electrical communication with connector 126 via aninternal coaxial cable 126 a (see FIG. 3A, for example) that extendsfrom the proximal end of the microwave antenna 100 and includes an innerconductor tip that is operatively coupled to a radiating section 138operably disposed within the shaft 112 and adjacent the conductive orradiating tip 114 (see FIG. 3A, for example). As is common in the art,internal coaxial cable 126 a is includes a dielectric material and anouter conductor surrounding each of the inner conductor tip anddielectric material. A connection hub (not shown) disposed at a proximalend of the microwave antenna 100 operably couples connector 126 tointernal coaxial cable 126 a, and cooling fluid supply 122 and a coolingfluid return 124 to a cooling assembly 120. Radiating section 138 by wayof conductive tip 114 (or in certain instances without conductive tip114) is configured to deliver radio frequency energy (in either abipolar or monopolar mode) or microwave energy (having a frequency fromabout 500 MHz to about 10 GHz) to a target tissue site. Elongated shaft112 and conductive tip 114 may be formed of suitable conductive materialincluding, but not limited to copper, gold, silver or other conductivemetals having similar conductivity values. Alternatively, elongatedshaft 112 and/or conductive tip 114 may be constructed from stainlesssteel or may be plated with other materials, e.g., other conductivematerials, such as gold or silver, to improve certain properties, e.g.,to improve conductivity, decrease energy loss, etc. In an embodiment,the conductive tip may be deployable from the elongated shaft 112. Inone particular embodiment, microwave antenna 100 may include anintroducer 116 having an elongated shaft 112 and a tip 114 that isnon-conductive. In this instance, the tip 114 may be made from anon-conductive material such as, for example, ceramic, plastic, etc.

With reference to FIG. 2, a schematic block diagram of the generator 200is illustrated. The generator 200 includes a controller 300 having oneor more modules (e.g., an ablation zone control module 332 (AZCM 332), apower supply 237 and a microwave output stage 238). In this instance,generator 200 is described with respect to the delivery of microwaveenergy. The power supply 237 provides DC power to the microwave outputstage 238 which then converts the DC power into microwave energy anddelivers the microwave energy to the radiating section 138 of themicrowave antenna 100. The controller 300 may include analog and/orlogic circuitry for processing sensed values provided by the AZCM 332and determining the control signals that are sent to the generator 200and/or supply pump 40 via a microprocessor 335. The controller 300 (orcomponent operably associated therewith) accepts one or more measuredsignals indicative of reflected power P_(r) associated with themicrowave antenna 100 when the microwave antenna is radiating energy.

One or more modules e.g., AZCM 332, of the controller 300 analyzes themeasured signals and determines if a threshold reflected power P_(r),e.g., P_(r1) has been met. If the threshold reflected power P_(r1) hasbeen met, then the AZCM 332, a microprocessor 335 and/or the controllerinstructs the generator 200 to adjust the microwave output stage 238and/or the power supply 237 accordingly. Additionally, the controller300 may also signal the supply pump to adjust the amount of coolingfluid to the microwave antenna 100 and/or the surrounding tissue. Thecontroller 200 includes microprocessor 335 having memory 336 which maybe volatile type memory (e.g., RAM) and/or non-volatile type memory(e.g., flash media, disk media, etc.). In the illustrated embodiment,the microprocessor 335 is in operative communication with the powersupply 237 and/or microwave output stage 238 allowing the microprocessor335 to control the output of the generator 300 according to either openand/or closed control loop schemes. The microprocessor 335 is capable ofexecuting software instructions for processing data received by the AZCM332, and for outputting control signals to the generator 300 and/orsupply pump 40, accordingly. The software instructions, which areexecutable by the controller 300, are stored in the memory 336.

One or more electrical properties (e.g., voltage, current, power,impedance, etc.) associated with a signal (or pulse) generated by thegenerator 200 may be monitored and measured. More particularly,electrical properties associated with a forward and reflected portion ofthe signal generated by the generator 200 is monitored and measured. Forexample, in one particular embodiment, forward and reflected power,P_(ƒ) and P_(r), respectively, of a signal for ablating tissue ismeasured by the AZCM 332, controller 300, microprocessor 337 or othersuitable module associated with the generator 200 and/or controller 200.

One or more control algorithms for predicting tissue ablation size isimplemented by the controller 300. More particularly, the concept ofcorrelating reflected power P_(r) associated with a particular microwaveantenna, e.g., the microwave antenna 100, with an ablation zone “A”having a radius “r” may be used to indicate tissue death or necrosis.More particularly, reflected power P_(r) associated with the microwaveantenna 100 varies over the course of an ablation cycle due to tissuecomplex permittivity changes caused by temperature increase (see FIGS.4A and 4B, for example). A relationship of reflected power P_(r) as afunction of time is represented by a control curve illustrated in FIG.4A. Likewise, a relationship of reflected power P_(r) as a function ofablation size is represented by a control curve illustrated in FIG. 4B.The control curves represented in FIGS. 4A and 4B are based on modelfunctions ƒ(t) and known measured values of reflected power P_(r) thathave been taken during an ablation procedure performed with themicrowave antenna 100, controller 300 and/or generator 200. Inaccordance with the present disclosure, the control curves depicted inFIGS. 4A and 4B (and/or equations mathematically associated therewith)may be utilized to calculate and/or verify when a specified thresholdreflected power P_(r) (e.g., reflected powers P_(r1-ss)) within aspecified time range (e.g., t₁-t_(ss)) not exceeding t_(ss), i.e., timewhen the microwave antenna 100 and ablated tissue is at a steady-statecondition, see FIG. 4A or FIG. 4B, for example. The significance of whenthe microwave antenna 100 and ablated tissue is at a steady-statecondition is described in greater detail below.

With reference now to FIGS. 4A and 4B, initially, an impedance mismatchbetween the microwave antenna 100 and tissue is present when themicrowave antenna 100 is inserted into uncooked tissue. This impedancemismatch is due to the 50 ohm impedance associated with the internalcable 126 a not matching the impedance of the radiating section 118and/or conductive tip 114. The impedance mismatch results in a non-zeroreflected power P_(ri), at the beginning of the ablation procedure, seeFIGS. 4A and 4B, for example. During the course of the ablationprocedure, tissue in a “near field” heats up resulting in a decrease inreflected power P_(r) (in a non-linear rate) (see FIG. 4A for example)until an optimal impedance match between the microwave antenna 100 andtissue is reached (see FIG. 4A at a time equal to time t₂ in combinationwith FIG. 4B at an ablation size having a radius “r” equal to 2 cm).That is, the total impedance Z_(t) of the microwave antenna 100 andtissue in the “near field” is approximately equal to 50 ohms. Themicrowave antenna 100 and tissue in the “near field” remain at thisoptimal impedance match for a brief period of time. At a time after timet₂, the microwave antenna 100 and tissue in the near field diverge fromthe optimal impedance match (in a non-linear rate). Ultimately, when themicrowave antenna 100 has heated tissue to a maximum attainabletemperature, an ablation zone “A” having a corresponding radius “r”(e.g., r_(ss)) is formed (see FIG. 3A in combination with FIGS. 4A and4B, for example). At this maximum temperature, a dielectric constant andconductivity associated with the ablated tissue reach a steady-statecondition (this steady-state condition occurs at time t_(ss)) thatcorresponds to a steady-state reflected power P_(rss) (hereinafterreferred to simply as P_(rss)) associated with the microwave antenna100. That is, because the ablated tissue is in a “near field” of themicrowave antenna 100, the ablated tissue essentially becomes part ofthe microwave antenna 100. Accordingly, when a dielectric constant andconductivity associated with the ablated tissue reaches a steady-statecondition, the reflected power P_(r) at the microwave antenna 100 alsoreaches a steady-state condition, e.g., P_(rss), FIG. 4A.

As noted above, the foregoing control algorithm includes one or moremodel functions ƒ(t) that are representative of the model curvesillustrated in FIGS. 4A and 4B. The model functions ƒ(t), model curvesdepicted in FIGS. 4A and 4B, and/or known measured values of reflectedpower P_(r), are utilized to obtain information relevant to thereflected power P_(r) such that real-time monitoring of an ablation zonemay be achieved. More particularly, a measurement of a slope of atangent line at a point along either of the control curves (e.g., curveillustrated in FIG. 4A) is equal to a derivative (dP_(r)/dt) of thecurve at that point. The calculation of the derivative at a particularpoint along the curve(s) provides information pertinent to the reflectedpower P_(r). More particularly, a rate of change of reflected powerP_(r) with respect to time and, more particularly, to a vector quantityof the rate of change of the reflected power P_(r) (i.e., direction(positive or negative) and magnitude of the rate of change) iscalculated from the control curve(s) depicted in FIGS. 4A-4C. This rateof change associated with reflected power P_(r) with respect to time maybe utilized, for example, to distinguish between a rise and fall of thereflected power P_(r). More particularly, points taken along the controlcurves depicted in FIGS. 4A and 4B correspond to values of reflectedpower P_(r), e.g., reflected powers P_(r1) and P_(r3), which correspondto ablation zones “A” having radii “r,” e.g., radii r₁ and r₃, atcorresponding times t, e.g., times t₁ and t₃. It should be noted that avalue of reflected power, e.g., P_(r1), corresponds to more than oneradius, e.g., r₁ and r₃ of an ablation zone.

More particularly, the representative control curve of reflected powerP_(r) depicted in FIGS. 4A and 4B illustrates reflected power P_(r)having an initial value, e.g., P_(i), at the beginning of an ablationprocedure. The reflected power P_(r) decreases until the reflected powerP_(r) is approximately equal to zero, i.e., when the total impedance ofmicrowave antenna 100 and tissue in a “near field” is approximatelyequal to 50 ohms. The reflected power P_(r) increases at a time aftertime t₂ when the total impedance of microwave antenna 100 and tissue inthe “near field” is not equal to 50 ohms. Accordingly, a measure ofreflected power P_(r) taken along the control curve provides one or morenumerical values of the reflected power P_(r) that is indicative of oneor more ablation zones “A” having corresponding radii “r.” For example,a measure of the reflected power P_(r), e.g., P_(r1), at time t₁corresponds to an ablation zone “A” having a radius r₁ that isapproximately equal to 1 cm, see FIGS. 4A and 4B collectively. Likewise,a measure of the reflected power P_(r), e.g., P_(r1), at time t₃ alsocorresponds to an ablation zone “A” having a radius r₃ that isapproximately equal to 2.2 cm, see FIGS. 4A and 4B collectively.

In accordance with the present disclosure, samples of a derivative takenat selective points along the control curve (e.g., points correspondingto radii r₁ and r₃ and/or points corresponding to times t₁ and t₃)provide information pertaining to the precise location (e.g., rise orfall portions of the control curve) of the reflected power P_(r) withrespect to the control curve.

More particularly, and for example, a derivative taken at a point alongthe control curve at time t₁ when the reflected power P_(r) isapproximately equal to P_(r1) is negative because the slope of thereflected power P_(r) is declining, as best seen in FIG. 4C. In thisinstance, reflected power P_(r1) may be thought of as having and isassigned a negative value indicating to one or more modules, e.g., AZCN332, associated with the controller 300 and/or generator 200 that thisvalue of the reflected power P_(r1) is indicative of and corresponds toan ablation zone “A” having a radius r₁. Similarly, samples of aderivative taken at a point along the control curve at time t₃ when thereflected power P_(r) is approximately equal to P_(r1) is positivebecause the slope of the reflected power P_(r) is increasing, as bestseen in FIG. 4C. In this instance, reflected power P_(r1) may be thoughtof as having and is assigned a positive value indicating to one or moremodules, e.g., AZCN 332, associated with the controller 300 and/orgenerator 200 that this value of the reflected power P_(r1) isindicative of and corresponds to an ablation zone “A” having a radiusr₃.

Implementing a control algorithm that utilizes a calculation of aderivative taken at a point on the control curve facilitates indetermining the precise size of the ablation zone “A.” That is, one ormore modules, e.g., AZCM 332, associated with the controller 300 andgenerator 200 is capable of distinguishing between which ablation zoneradius “r,” e.g., radius r₁ or r₂, corresponds to the reflected powerP_(r), e.g., measured reflected power P_(r1). Moreover, in the instancewhere multiple ablation zones “A” are located adjacent to one another,tissue impedance of uncooked tissue at a near field of an ablation zone“A” may effect a reflected power P_(r) measurement. More particularly,tissue impedance of uncooked tissue at the near field may be slightlyhigher or lower (depending on a specific adjacent ablation zone “A”),which, in turn, may cause the reflected power P_(r) to be higher orlower at the beginning of an ablation procedure then is expected. Thus,in the instance where the initial reflected power P_(i) is approximatelyequal to P_(r4), a calculation of the derivative taken at a point on thecontrol curve indicates that the microwave antenna 100 is positionedadjacent cooked or ablated tissue. That is, the initial positive valueof P_(r4) indicates that the reflected power P_(r) is increasing, and,thus, a steady-state condition is approaching, i.e., a calculation ofthe derivative indicates that the measured reflected power P_(r) is inthe rising portion of the control curve and the reflected power P_(r)will not approach zero, i.e., a point on the control curve where thetotal impedance associated with the microwave antenna 100 and tissueadjacent the near field is approximately equal to 50 ohms.

The microwave antenna 100 of the present disclosure may be configured tocreate an ablation zone “A” having any suitable configuration (e.g., awidth “w” and a length “l”), such as, for example, spherical (FIG. 3A),hemispherical, ellipsoidal (FIG. 3B where the ablation zone isdesignated “A-2”), and so forth. In one particular embodiment, microwaveantenna 100 is configured to create an ablation zone “A” that isspherical (FIG. 3A). As noted above, when the microwave antenna 100 hasheated tissue in the “near field” to a maximum temperature, a dielectricconstant and conductivity associated with the ablated tissue reaches asteady-state that corresponds to a steady-state reflected power P_(rss)associated with the microwave antenna 100. Correlating the P_(rss)associated with the microwave antenna 100 with the ablated tissue (i.e.,ablated tissue, where the dielectric constant and conductivity are in asteady-state condition), indicates a specific size (e.g., radius r_(ss))and shape (e.g., spherical) of the ablation zone “A.” Thus, a measure ofP_(rss) associated with the microwave antenna 100 corresponds to anablation zone “A” having a radius r, e.g., r_(ss). The control algorithmof the present disclosure uses known steady-state reflected powersassociated with specific microwave antennas at specific radii to predictan ablation size. That is, reflected powers P_(r), e.g., P_(rss),associated with a specific microwave antenna, e.g., microwave antenna100, and corresponding radius, e.g., r_(ss), are compiled into one ormore look-up tables “D” and are stored in memory, e.g., memory 336,accessible by the microprocessor 335 and/or the AZCM 332. Thus, when ameasured reflected power for a specific microwave antenna, e.g.,microwave antenna 100, reaches P_(rss) one or more modules, e.g. AZCM332, associated with the controller 300, commands the controller 200 toadjust the power output to the microwave antenna 100 accordingly. Thiscombination of events will provide an ablation zone “A” with a radiusapproximately equal to r_(ss).

In an embodiment, for a given microwave antenna, e.g., microwave antenna100, reflected power measurements may be taken at times prior to t_(ss),e.g., times t₁-t₄. In this instance, reflected powers, e.g.,P_(r1)-P_(r4), associated with the microwave antenna 100 may becorrelated with an ablation zone “A” defined by a plurality ofconcentric ablation zones having radii r₁-r₄ (collectively referred toas radii “r”) when measured from the center of the ablation zone “A.”More particularly, the reflected powers P_(r1)-P_(r4) and correspondingradii “r” may be correlated with each other in a manner as describedabove with respect to P_(rss) and r_(ss) (see FIG. 3A in combinationwith FIGS. 4A and 4B, for example). In this instance, when specificreflected power, e.g., P₃, is met one or more modules, e.g. AZCM 332,associated with the controller 300, commands the controller 200 toadjust the power output to the microwave antenna 100 accordingly.

It should be noted, that a reflected power P_(r) associated with amicrowave antenna 100 may vary for a given microwave antenna. Factorsthat may contribute to a specific reflected power P_(r) for a givenmicrowave antenna include but are not limited to: dimensions associatedwith the microwave antenna (e.g., length, width, etc.); type of materialused to manufacture the microwave antenna (or portion associatedtherewith, e.g., a radiating section) such as copper, silver, etc; andthe configuration of the radiating section (e.g., dipole, monopole,etc.) and/or a conductive tip (e.g., sharp, blunt, curved, etc)associated with the microwave antenna. Other factors that may contributeto a specific reflected power P_(r) for a given microwave antenna mayinclude, for example, type of microwave antenna (e.g., microwave antennaconfigured for use in treating lung, kidney, liver, etc.), type oftissue being treated (e.g., lung, kidney, liver, heart etc.), tumorsize, and so on.

AZCM 332 may be a separate module from the microprocessor 335, or AZCM332 may be included with the microprocessor 335. In an embodiment, theAZCM 332 may be operably disposed on the microwave antenna 100. The AZCM332 may include control circuitry that receives information from one ormore control modules and/or one or more impedance sensors (not shown),and provides the information to the controller 300 and/or microprocessor335. In this instance, the AZCM 332, microprocessor 335 and/orcontroller 300 may access look-up table “D” and confirm that aparticular reflected power (e.g., P_(rss)) associated with microwaveassembly 100 corresponds to a specific ablation zone (e.g., specificablation zone having a radius r_(ss)) has been met and, subsequently,instruct the generator 200 to adjust the amount of microwave energybeing delivered to the microwave antenna. In one particular embodiment,look-up table “D” may be stored in a memory storage device (not shown)associated with the microwave antenna 100. More particularly, a look-uptable “D” may be stored in a memory storage device operativelyassociated with handle 118 and/or connector 126 of the microwave antenna100 and may be downloaded, read and stored into microprocessor 335and/or memory 336 and, subsequently, accessed and utilized in a mannerdescribed above; this would do away with reprogramming the generator 200and/or controller 300 for a specific microwave antenna. The memorystorage device may also be configured to include information pertainingto the microwave antenna 100. Information, such as, for example, thetype of microwave antenna, the type of tissue that the microwave antennais configured to treat, the type of ablation zone desired, etc. may bestored into the storage device associated with the microwave antenna. Inthis instance, for example, generator 200 and/or controller 300 ofsystem 10 may be adapted for use with a microwave antenna configured tocreate an ablation zone, e.g. ablation zone “A-2,” different from thatof microwave antenna 100 that is configured to create an ablation zone“A.”

In the embodiment illustrated in FIGS. 1-4, the generator is shownoperably coupled to fluid supply pump 40. The supply pump 40 is, inturn, operably coupled to the supply tank 44. In embodiments, themicroprocessor 335 is in operative communication with the supply pump 40via one or more suitable types of interfaces, e.g., a port 240operatively disposed on the generator 200, which allows themicroprocessor 335 to control the output of a cooling fluid from thesupply pump 40 to the microwave antenna 100 according to either openand/or closed control loop schemes. The controller 300 may signal thesupply pump 40 to control the output of the cooling fluid from thesupply tank 44 to the microwave antenna 100. In this way, cooling fluid42 is automatically circulated to the microwave antenna 100 and back tothe supply pump 40. In certain embodiments, a clinician may manuallycontrol the supply pump 40 to cause cooling fluid 42 to be expelled fromthe microwave antenna 100 into and/or proximate the surrounding tissue.

Operation of system 10 is now described. In the description thatfollows, it is assumed that losses associated with the connector 126and/or cable 162 a are negligible and, thus, are not needed incalculating and/or determining a reflected power of the microwaveantenna 100 adjacent the ablation zone during the ablation procedure.Initially, microwave antenna 100 is connected to generator 200. In oneparticular embodiment, one or more modules, e.g., AZCM 332, associatedwith the generator 200 and/or controller 300 reads and/or downloads datafrom a storage device associated with the antenna 100, e.g., the type ofmicrowave antenna, the type of tissue that is to be treated, etc.Microwave antenna 100 may then be positioned adjacent tissue (FIG. 3A).Thereafter, generator 200 may be activated supplying microwave energy toradiating section 138 of the microwave antenna 100 such that the tissuemay be ablated. During tissue ablation, when a predetermined reflectedpower, e.g., P_(rss), at the microwave antenna 100 is reached, the AZCM332 instructs the generator 200 to adjust the microwave energyaccordingly. In the foregoing sequence of events the AZCM 332 functionsin real-time controlling the amount of microwave energy to the ablationzone such that a uniform ablation zone of suitable proportion (e.g.,ablation zone “A” having a radius r_(ss)) is formed with minimal or nodamage to adjacent tissue.

With reference to FIG. 5 a method 400 for monitoring temperature oftissue undergoing ablation is illustrated. At step 402, microwave energyfrom generator 200 is transmitted to a microwave antenna 100 adjacent atissue ablation site. At step, 404 reflected power P_(r) associated withthe microwave antenna is monitored. At step 406, a detection signal iscommunicated to the generator 200 when a predetermined reflected powerP_(r) is reached at the microwave antenna 100. At step 408, the amountof microwave energy from the generator 200 to the microwave antenna 100may be adjusted.

From the foregoing and with reference to the various figure drawings,those skilled in the art will appreciate that certain modifications canalso be made to the present disclosure without departing from the scopeof the same. For example, one or more directional couplers (not shown)may be operatively associated with the generator 200, controller 300and/or AZCM 332, and configured to direct the forward, reflected, and/orload power portions of a sampled output signal (or pulse) to the AZCM332. More particularly, the directional coupler provides samples of theforward and reflected signal (or pulse) generated by the generator 200.The power, magnitude and phase of the generated output signal may beobtained or calculated from the measured forward and reflected signalsby conventional algorithms that employ one or more suitable equations.

It should be noted that energy values or parameters (e.g., power,voltage, current, impedance, magnitude and phase) of an output pulse arevalid at the output of generator 200. That is, and as alluded to above,the connector 126 and/or internal cable 126 a may include transmissionline losses. Accordingly, in order to get a more accurate reading and/ormeasurement of the energy values or parameters that are delivered to themicrowave antenna 100 and/or reflected back to the generator 200, onewould have to know the actual transmission line losses associated withconnector 126 and/or internal cable 126 a. Accordingly, in anembodiment, loss information for connector 126 and/or internal cable 126a may be determined and, subsequently, stored in memory 336 and accessedby one or more modules, such as, for example, a calibration module (600)or other suitable module (e.g., AZCM 332) for later use. The lossinformation for connector 126 and/or internal cable 126 a may bedetermined by any suitable device and/or method. For example, the lossinformation for connector 126 and/or internal cable 126 a may bedetermined via network analyzer 602. In one particular embodiment, thenetwork analyzer 602 may be an integral part of generator 200 (e.g.,part of calibration module 600) or, alternatively, the network analyzer602 may be a separate handheld device that is in operative communicationwith generator 200. The network analyzer 602 may be used to perform adiagnostic test of connector 126 and/or internal cable 126 a. Thenetwork analyzer 602 may function in a fashion similar to mostconventional network analyzers that are known in the available art. Thatis, the network analyzer 602 may determine the properties that areassociated with connector 126 and/or internal cable 126 a, and moreparticularly, those properties that are associated with connector 126and/or internal cable 126 a that affect the reflection and/ortransmission of an output signal, such as, for example, thecharacteristic impedance Z_(o) of connector 126 and/or internal cable126 a.

Known line loss information associated with the connector 126 and/orinternal cable 126 a may be stored into memory 336 and accessed duringan ablation procedure by one or more modules, e.g., AZCM 332, associatedwith the controller 300 and/or generator 200 and, subsequently, used indetermining if a predetermined threshold value, e.g., P_(r1-ss), of thereflected power P_(r) has been met. More particularly, characteristicimpedance associated with connector 126 and/or internal cable 126 a maybe employed to determine a more accurate or comprehensive measurement ofthe reflected power P_(r). For example, a more accurate or comprehensivemeasurement of the reflected power P_(r) may be determined using theequation:

$\begin{matrix}{\frac{Z_{1 - {ss}} - Z_{o}}{Z_{1 - {ss}} + Z_{o}} = \frac{P_{SWR} - 1}{P_{SWR} + 1}} & (1)\end{matrix}$

where, Z_(o) is the characteristic impedance associated with theconnector 126 and/or internal cable 126 a, Z_(1-ss) is an impedance ofthe microwave antenna 100 when the microwave antenna 100 is positionedadjacent tissue in a “near field” at times t_(1-ss), and P_(swr) is apower standing wave ratio (P_(swr)) that may be calculated using theequation:

$\begin{matrix}{P_{SWR} = \frac{P_{f} + P_{r}}{P_{f} - P_{r}}} & (2)\end{matrix}$

where P_(ƒ) is the power associated with the generated signal (i.e.,forward signal) and P_(r) is the power associated with the reflectedsignal. The characteristic impedance Zo is an accurate measure of theimpedance of the connector 126 and/or internal cable 126 a and takesinto account the line losses associated with the connector 126 and/orinternal cable 126 a. In this instance, after all the necessarycalculations have been carried out, an accurate representation of thereflected power P_(r) may be transmitted to and measured by the AZCM 332(or other suitable module associated with either the controller 300 orgenerator 200).

While several embodiments of the disclosure have been shown in thedrawings and/or discussed herein, it is not intended that the disclosurebe limited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Therefore, the above description should not be construed as limiting,but merely as exemplifications of particular embodiments. Those skilledin the art will envision other modifications within the scope and spiritof the claims appended hereto.

1-20. (canceled)
 21. A method for monitoring tissue undergoing ablation,the method comprising: transmitting microwave energy from a power sourceto a microwave antenna to form a tissue ablation zone; monitoringreflected power associated with the microwave antenna as the tissueablation zone forms; communicating a control signal to the power sourcewhen a predetermined reflected power is reached at the microwaveantenna; and adjusting a property of microwave energy from the powersource to the microwave antenna, wherein the microwave antenna isconfigured to produce a spherical ablation zone.
 22. The methodaccording to claim 21, further comprising: accessing at least one datalook-up table including data pertaining to a control curve varying overtime and being representative of at least one electrical parameterassociated with the microwave antenna, points along the control curvecorresponding to a value of the at least one electrical parameter; andgenerating a signal when a predetermined threshold value of the at leastone electrical parameter is corresponding to a radius of the tissueablation zone.
 23. The method according to claim 22, wherein the atleast one electrical parameter is selected from the group consisting ofimpedance, power, voltage, and current.
 24. The method according toclaim 23, further comprising correlating a reflected signal generated bythe power source associated with the microwave antenna duringtransmission of microwave energy to the microwave antenna when themicrowave antenna is in a near field state with the tissue ablationzone.
 25. The method according to claim 24, further comprisingcalculating a reflected power associated with the microwave antenna. 26.The method according to claim 25, further comprising calculating aderivative at a point taken along the control curve corresponding to thereflected power associated with the microwave antenna.
 27. A method formonitoring tissue undergoing ablation, the method comprising:transmitting microwave energy from a power source to a microwave antennato form a tissue ablation zone; monitoring reflected power associatedwith the microwave antenna; calculating a derivative at a point takenalong a control curve corresponding to the reflected power; anddetermining whether the microwave antenna and tissue in a near field ofthe tissue ablation zone are approaching a respective steady-statecondition based on the derivative.
 28. The method according to claim 27,wherein monitoring reflected power includes monitoring reflected powerassociated with the microwave antenna to generate a reflected powersignal.
 29. The method according to claim 28, wherein calculating thederivative includes calculating a derivative indicative of rise and fallportions of the reflected power signal, wherein when the derivative istaken during a rise portion of the reflected power signal the reflectedpower is assigned a positive value and when the derivative is takenduring a fall portion of the reflected power signal the reflected poweris assigned a negative value, the positive and negative values of thereflected power signal indicating when the microwave antenna and tissuein a near filed of the tissue ablation zone are approaching a respectivesteady-state condition or an impedance match between the microwaveantenna and tissue in the near field.
 30. The method according to claim28, wherein monitoring reflected power includes generating the reflectedpower signal in response to the reflected power reaching a predeterminedthreshold value corresponding to a radius of the tissue ablation zoneformed by the microwave antenna.
 31. The method according to claim 27,further comprising: accessing at least one data look-up table includingdata pertaining to a control curve varying over time and beingrepresentative of at least one electrical parameter associated with themicrowave antenna; and generating a signal when a predeterminedthreshold value of the at least one electrical parameter iscorresponding to a radius of the tissue ablation zone.
 32. The methodaccording to claim 31, wherein the at least one electrical parameter isselected from the group consisting of impedance, power, voltage, andcurrent.
 33. The method according to claim 32, further comprisingcorrelating a reflected signal generated by the power source associatedwith the microwave antenna during transmission of microwave energy tothe microwave antenna when the microwave antenna is in a near fieldstate with the tissue ablation zone.